1. Field of the Invention
This invention relates to a recording disc for information signals, such as an optical disc or a magneto-optical disc, and a disc chucking apparatus for chucking the recording disc to a disc recording and/or reproducing apparatus.
2. Description of Related Art
A recording disc for recording and/or reproducing information signals, such as an optical disc or a magneto-optical disc, has heretofore been known and put to practical use. Referring to FIG. 1, the recording disc includes a disc substrate 201, formed of a light-transmitting material, such as polycarbonate resin, and a recording layer, such as a layer of a metal or a magnetic material, is deposited on a major surface of the disc substrate 201. A light beam is irradiated on the other major surface of the substrate, that is the major surface thereof opposite to the surface on which the recording layer is deposited for recording or reproducing the information signals on or from the recording layer.
The disc substrate 201 is formed with a center hole 202 to permit the disc to be held by a recording and/or reproducing apparatus adapted for recording and/or reproducing the information signals on or from the recording disc. The center hole 202 is formed in the disc substrate 201 so that the center of the center hole 202 is in register with the center of curvature of recording track(s) formed concentrically or spirally on the recording layer.
The recording disc is set on a disc table 204 in the recording and/or reproducing apparatus. The disc table 204 is in the form of a disk larger in diameter than the center hole 202 and is mounted on the distal end of a shaft 203 of a spindle motor, not shown. A setting portion 204a, on which the disc substrate 201 of the recording disc is set, is formed on the upper peripheral surface of the disc table 204, that is on the major surface of the disc table 204 situated in the direction of projection of the distal end of the driving shaft 203.
A centering member 205 is reciprocably mounted at the center of the disc table 204. The centering member 205 is in the form of a cone gradually tapered towards the distal end of the rotating shaft 203 and has an outside diameter substantially corresponding to the diameter of the center hole 202. The centering member 205 is elastically biased in the direction of the distal end of the rotating shaft 203 by a compression coil spring, not shown.
When loaded in the recording and/or reproducing apparatus, the recording disc is set on the disc table 204 and is thrust, by a chucking unit, not shown, towards the disc table 204, so that the mid part of the major surface of the disc is abutted against the setting portion 204a. That is, with the recording and/or reproducing apparatus, a so-called self alignment or centering operation is achieved by the inner periphery of the center hole 202 sliding on the outer peripheral surface of the centering member 205.
The recording disc may be self-aligned in this manner even if it is offset with respect to the centering member 205 by a distance corresponding to the width of the outer peripheral surface 205a of the centering member 205 along the major surfaces of the disc substrate 201, as indicated by dimension b in FIGS. 1 and 2, as disclosed for example in U.S. Pat. No. 4,340,955.
If, in the recording and/or reproducing apparatus, the recording disc is to be held more reliably and satisfactorily, the self-alignment or centering operation needs to be achieved even if the amount of offset of the recording disc with respect to the centering member 205 is increased when the recording disc is set on the disc table 204. To this end, the width of the outer peripheral surface 205a of the centering member 205 along the major surface of the disc substrate 201 needs to be increased.
However, if the width of the outer peripheral surface 205a of the centering member 205 along the major surface of the disc substrate 201 is increased in this manner, the slant of the outer peripheral surface 205a with respect to the major surfaces of the disc substrate 201 becomes more gentle, so that the inner peripheral surface of the center hole 202 becomes more difficult to slide along the outer peripheral surface 205a. The result is that the amount of shifting of the disc for centering is insufficient so that a satisfactory self-alignment or centering is not achieved.
Thus, for achieving satisfactory self-alignment, it is necessary to increase the width of the outer peripheral surface 205a in a direction along the major surfaces of the disc substrate 201, while maintaining the slant of the surface 205a with respect to the major surfaces of the disc substrate 201. To this end, it is necessary to increase the height, shown by an arrow h in FIG. 2, of the surface 205a in a direction normal to the major surfaces of the disc substrate 201.
If the height of the surface 205a in the direction normal to the major surfaces of the disc substrate 201 is increased, the centering member 205 is protruded on the major surface of the disc substrate 201 opposite to the disc table setting major surface via the disc substrate when the disc is set on the disc table 204. If the centering member 205 is protruded in this manner on the major surface of the disc substrate opposite to the disc table setting major surface when the disc is held by the recording and/or reproducing apparatus, the disc rotating unit inclusive of the disc table is increased in height, so that the recording and/or reproducing apparatus is correspondingly increased in size.
On the other hand, when the recording disc is housed in a thin casing-like disc cartridge, there is a risk of the centering member 205 being abutted against the inner surface of a cartridge body of the disc cartridge when the recording disc is loaded on the disc table 204. In order for the centering member 205 not to be protruded towards the major surface of the disc substrate 201 opposite to the disc table setting major surface to thereby avoid the contact of the centering member with the inner surface of the cartridge body, the cartridge body needs to be increased in size, thus leading to an increased size of the disc cartridge.